a. Technical Field
The present invention is in the field of digital potentiometers.
b. Discussion of the Related Art
Digital potentiometers are electronic circuits that can provide a variable impedance as a result of processing a digital sequence. A digital potentiometer has a fixed-value impedance connected between two reference voltage terminals. This impedance is provided by a string of impedance units that can be selectively connected to a third terminal, called the wiper terminal herein, through electronic switches controlled by digital signals. Digital potentiometers can be used in digital to analog converters, and as replacements for mechanical potentiometers and rheostats.
The fineness of adjustment, resolution, or “granularity” of a digital potentiometer is typically determined by the number of digital bits used for the selection of the desired wiper position. For instance, a three-bit wiper address (n=3) allows for 23 (i.e., 8) different wiper impedance selections, i.e., wiper positions.
FIG. 1 is a schematic diagram of a digital potentiometer 100 that operates on the voltage-scaling principle. A string 101 of 2n−1 resistors R0, R1, . . . R2n−2 is connected between a high reference voltage (VREF+) terminal 102 and a low reference voltage (VREF−) terminal 104. The voltage drop across each one of the resistors is equal to one least significant bit (LSB) of output voltage Vw change. The output analog voltage signal Vw is collected on wiper terminal 106.
The wiper position is set by a switch decoding network, illustrated by decoder 108 and 2n wiper switches S0, S1, . . . S2n−1. The decoder 108 receives an n-bit wiper address on line 110, decodes the n-bit wiper address, and outputs 2n one-bit logical control signals C0, C1, . . . C2n−1 on lines 112(0)–112(2n−1). A respective one of the control signals is provided to each respective one of the 2n wiper switches S0, S1, . . . S2n−1. Each of the 2n wiper switches S0, S1, . . . S2n−1 taps a different node in the resistor string 101. The state of the particular control signal C0–C2n−1 received by the respective wiper switch S0–S2n−1 determines whether the wiper switch will be open (i.e., nonconducting) or closed (i.e., conducting). Closing a selected one of wiper switches while leaving all of the other wiper switches open provides a unique ratio between the resistance values of the two resistor sub-chains connected to wiper terminal 106 via the closed wiper switch.
One disadvantage of this configuration is the relatively large number of components necessary to implement the digital potentiometer, including the 2n−1 resistors of string 101, the 2n wiper switches, and the many components of logic decoder 108. For example, in a four bit (n=4) implementation, the digital potentiometer 100 requires 15 resistors, 16 wiper switches, and a decoder 108 large enough to control the 16 wiper switches. In an eight-bit (n=8) implementation, the digital potentiometer 100 requires 255 resistors, 256 wiper switches, and a decoder 108 large enough to control the 256 wiper switches.
In general, it is desirable to reduce the number of components in a digital potentiometer for purposes of die-area savings, higher manufacturing yields, lower costs, and improved electrical performance.